When optical fibers are fusion spliced, protective coating at the connecting end portions of the optical fibers is removed to expose glass fiber and the end portions of the glass fiber are fused together. Since the portion where the protective coating has been removed has a low mechanical strength, it is reinforced with a protection sleeve covering it. The protection sleeve has a heat shrinkable tube (outer tube) that can shrink in a radial direction when heated and a reinforcing rod and an adhesive tube (inner tube) made of a hot-melt adhesive resin housed inside the outer tube.
FIG. 7A is a schematic view of a protection sleeve 1 disclosed in Japanese Laid-open Patent Publication No. 2002-347721 (Patent Document 1). The protection sleeve 1 has a heat shrink outer tube 2 and a hot-melt inner tube 3 and a reinforcing rod 4 housed inside the outer tube 2. A portion 5 at each of both ends of the outer tube 2 is heat-shrunk so as to hold the reinforcing rod 4 and the inner tube 3 is held by friction.
FIG. 7B is a schematic view illustrating a manufacturing method for the protection sleeve 1. A plurality of the outer tube 2 each of which accommodates the inner tube 3 and the reinforcing rod 4 are carried successively at a prescribed speed by a belt conveyer 7 in a state where the reinforcing rod 4 is positioned downward. The end portions of the outer tube 2 is selectively heated by a heating element 6 which is arranged in a position along the path through which the protection sleeve 1 is carried, whereby the outer tube heat shrinks at the end portions and the reinforcing rod 4 is secured in the outer tube 2.
FIG. 7C is a schematic view illustrating a configuration in which the protection sleeve 1 is used. One of the optical fibers 8a and 8b that are to be fusion spliced together is first inserted into the protection sleeve 1. The protective coating on the end portions of the optical fibers 8a and 8b are removed and the optical fibers 8a and 8b are fusion spliced at the mating ends 9b of the exposed bare fibers 9a. The protection sleeve 1 is then arranged to a position where the fusion spliced portion locates at the middle position of the sleeve such that it covers the bare fibers 9a and the adjacent fiber coating. Next, the entire protection sleeve 1 is heated for a prescribed amount of time in order to shrink the outer tube 2 and to melt the inner tube 3 such that it fills the space inside the outer tube 2. Afterwards, the inner tube 3 hardens and the protection sleeve and optical fiber become secured together as an integral unit, thereby reinforcing and protecting the fusion spliced portion of the optical fiber.
With the protection sleeve 1, if the heating amount is unstable and the shrinkage of the end portions is insufficient, then the reinforcing rod will easily become dislodged. Also, since both ends are shrunk in advance on the side where the reinforcing rod is located, the shrinkage rate on the side where the reinforcing rod is located will become smaller than the shrinkage rate of the side where the optical fiber is located when the protection sleeve covering the fusion spliced portion of the optical fiber is heated and the optical fiber is possibly to be bend.
Japanese Laid-open Patent Publication No. 11-52163 (Patent Document 2) discloses a protection sleeve which is heat-shrunk at an intermediate portion thereof so as to hold a reinforcing rod. With this protection sleeve, the bending of the optical fiber that occurs with the protection sleeve 1 can be avoided, but the holding force obtained with respect to the reinforcing rod is weak and the reinforcing rod can easily become dislodged.